1. Field of the Invention
The present invention relates to a field-emission transmission electron microscope. To be more precise, it relates to a field-emission transmission electron microscope comprising an electrostatic lens for extracting electrons from a field-emission cathode and for accelerating the electrons with a predetermined accelerating voltage. Additionally, a condenser lens for focusing the accelerated electron beam and illuminating a specimen with the focused electron beam is provided. The invention relates, in particular, to the constitution and the operation method of the condenser lens portion.
2. Description of the Related Art
In the case of a conventional transmission electron microscope, for example, a thermionic electron source in which thermionic electrons are emitted from tungsten W or LaB.sub.6 being heated, or a field-emission electron source in which electrons are extracted from a hairpin cathode having a small radius of curvature with a strong electric field applied to it, is used as an electron source for generating an electron beam.
In the case of an electron microscope using a thermionic electron source, an electron source diameter of the thermionic electron source is as large as 2 to 10 .mu.m, and in an observation with high magnification, in order to make an area on a specimen to be illuminated by an electron beam be less than 1 .mu.m, the magnification of the illumination system shall be less than 1. An example of such a case is described in a Japanese Patent laid open under Provisional Publication No. 126951/80.
In the case of an electron microscope using a field-emission electron source, the illumination system is constituted with an electrostatic lens and a single condenser lens. An example of such a case is described in Japanese Patent Publication No. 117534/85.
In recent years, there has been a demand for a transmission electron microscope having functions both of observation of high resolution imaging and of microanalysis. In a conventional transmission electron microscope using a thermionic electron source, the magnification of an illumination system becomes less than 1 in both cases. Because the size of a thermionic electron source is as large as 2 to 10 .mu.m, reduction of the source size is necessary to prevent a decrease in the brightness of an image in the case of an observation with high magnification, and reduction of the source size to a large extent is needed in the case of microanalysis. However, because of the insufficiency in the brightness of an electron source, it is almost impossible to make the diameter of an electron beam on a specimen less than 10 nm in order to obtain a necessary probe current for an analysis.
On the other hand, in the case of a field-emission transmission electron microscope mounted with a field-emission electron source which has higher brightness and a smaller source size than the case of a thermionic electron source, there is a possibility of having a function of analyzing a small area and a function of an observation of high resolution imaging. In order to realize these two functions, observation of high resolution imaging and microanalysis, it is necessary to make the magnification of an illumination system less than 1 in the case of microanalysis, and to make the magnification of the illumination system more than 1 in the case of observation of high resolution imaging. In other words, in the case of a field-emission electron source, the electron source size is as small as about 10 nm; however, in order to perform a microanalysis of a small area on a specimen in reducing an electron beam diameter, for example, down to less than 1 nm on the specimen, it is necessary to make the magnification of the illumination system be less than 1/10. On the other hand, it is necessary to make an illuminating angle on a specimen small to obtain a high resolution electron microscopic image.
As shown in FIG. 2, let us assume that an electron beam extracted at an extracting voltage of V1 from a field-emission electron source 1 is accelerated to an accelerating voltage V0 by an electrostatic lens 2 and is focused on a specimen 7 by a condenser lens 8; let a symbol .alpha. indicate an exit half angle of an electron source to be limited by an aperture 9 and let a symbol .beta. indicate an illuminating angle on a specimen, then the magnification of the illumination system M is expressed as, according to the Helmholtz equation, EQU M=.alpha./.beta. (V1/V0).sup.0.5.
Let a symbol .omega. indicate the angular current density (emission current per unit solid angle) of the field-emission cathode 1, then a beam current I to be limited by the aperture 9 is expressed as I=.pi..alpha..sup.2 .omega..
When tungsten W of bearing [310] is used as the field-emission cathode 1, the maximum angular current density .omega. is about 50 .mu.A/sr. In order to obtain a bright enlarged image on a fluorescent screen, a beam current of the order of I=4nA is necessary, and the symbol .alpha. to satisfy the condition is calculated to be .alpha.=5 mrad. An extracting voltage is usually in the range of 4 kV to 6 kV. Assuming that an extracting voltage V1=6 kV, accelerating voltage V0=200 kV, and .alpha.=5 mrad, the magnification M of the illumination system is EQU M=0.86/.beta. (where the unit of .beta. is mrad).
Therefore, in order to obtain an illuminating angle .beta. of less than 0.5 mrad being necessary for a high resolution observation, the magnification M has to be made as large as possible.
In a conventional example, however, a condenser lens 8 is a single condenser lens, so that it is impossible to change magnification by a large extent from reduction to enlargement using the condenser lens 8. Because of this, when a lens is disposed in a position capable of obtaining an electron beam of less than 1 nm (suitable for microanalysis) in an observation with high magnification, sufficiently large magnification cannot be obtained; thereby, an illuminating angle of less than 0.5 mrad cannot be obtained, which makes it impossible to perform a high resolution observation. In contrast with this, when a lens is disposed in a position to be able to obtain a large magnification, microanalysis of a minute portion of less than 1 nm becomes impossible. Therefore, in the case of an example of a conventional field-emission transmission electron microscope, it is impossible to deal with both cases, high resolution observation and microanalysis, with a single condenser lens.